I like having fully-spanning wallpaper on my three screens. 3840x1024 images are what you need, but getting a detailed photo at that resolution is hard (you really need a ~20MPx camera with a very expensive lens). Here are some that I’ve made, mostly using a 10MPx camera, occasionally with some stitching.

I make no claims as to any artistic or technical merit in any of these.

Neuschwanstein Castle. Composite of about 10 10MPx photos (with a wobbly horizon) so the detail is very good.